The present invention relates to improved circuit breaker drawout apparatus for electrical switchboards and particularly to drawout apparatus capable of withstanding extreme blowout forces developed during high fault current interruptions by the breaker.
Commonly assigned U.S. Pat. No. 4,004,113 discloses circuit breaker drawout apparatus for electrical switchboards wherein a circuit breaker is mounted by a carriage which, in turn, is supported on telescoping rails for movement between extended and engaged positions through intermediate, disengaged and test positions. For movement into and out of the engaged position during which rather sizable contact pressures of the switchboard primary disconnect contacts must be contended with, the mechanical advantage afforded by a racking mechanism is utilized. This racking mechanism includes a racking lead screw rotatably mounted to one side of the carriage for translating a travelling nut drivingly connected to swing a crank through a predetermined arc. This crank carries a crank pin which moves through an elongated cam slot provided in a switchboard mounted cam plate fixedly mounted to one side of the switchboard cubicle pursuant to camming the circuit breaker into and out of its engaged position. To ensure non-skewed racking movement of the circuit breaker, the crank drivingly connected with the travelling nut is mounted on one end of a crank shaft journaled by the carriage and extending transversely behind the breaker to the other side of the carriage. A second crank is mounted on this other end of the crank shaft and carries a crank pin operating in an indentically configured cam slot provided in a second cam plate mounted to the other side of the switchboard cubicle. Thus, the two cranks swing in unison to develop substantially equal camming forces at each side of the carriage for evenly racking the circuit breaker into and out of its engaged position.
It has been discovered that the electromotive forces produced by high fault currents flowing through the breaker, coupled with the gaseous high pressure exhaust emanating from the breaker while interrupting such currents, develops blowout forces of greater than anticipated magnitudes acting in a direction to drive the breaker away from its engaged position. Since the primary disconnect contact pressures have very little holding power in opposition to such blowout forces, the drawout apparatus must absorb the brunt if the breaker engaged position is to be sustained. Obviously, separation of the primary contacts while high fault currents are flowing therethrough produces violent arcing which, at the very least, will destroy the breaker and switchboard disconnect contacts.
It is accordingly an object of the present invention to provide improved circuit breaker drawout apparatus capable of withstanding the tremendous blowout forces attending a high fault current interruption.
An additional object is to provide drawout apparatus of the above character wherein the circuit breaker is positively latched in its engaged position.
A further object is to provide drawout apparatus of the above character wherein the circuit breaker is latched in its engaged position incident with racking of the breaker thereto and is unlatched incident with racking of the breaker therefrom.
Another object is to provide drawout apparatus of the above character wherein the engaged position latching provision is implemented in a simple, practical and effective manner and is convenient to operate.
Other objects of the invention will in part be obvious and in part appear hereinafter.